Micro-gyroscopes are used in many applications including, but not limited to, communications, control and navigation systems for both space and land applications. These highly specialized applications need high performance and cost effective micro-gyroscopes.
There is known in the art a micro-machined electromechanical vibratory gyroscope designed for micro-spacecraft applications. The gyroscope is explained and described in a technical paper entitled “Silicon Bulk Micro-machined Vibratory Gyroscope” presented in June, 1996 at the Solid State Sensors and Actuator Workshop in Hilton Head, S.C.
The prior art gyroscope has a resonator having a “cloverleaf” structure consisting of a rim, four silicon leaves, and four soft supports, or cantilevers, made from a single crystal silicon. A metal post is rigidly attached to the center of the resonator, in a plane perpendicular to the plane of the silicon leaves, and to a quartz base plate with a pattern of electrodes that coincides with the cloverleaf pattern of the silicon leaves. The electrodes include two drive electrodes and two sense electrodes.
The micro-gyroscope is electrostatically actuated and the sense electrodes capacitively detect Coriolis induced motions of the silicon leaves. The response of the gyroscope is inversely proportional to the resonant frequency and a low resonant frequency increases the responsivity of the device.
Micro-gyroscopes are subject to electrical interference that degrades performance with regard to drift and scale factor stability. Micro-gyroscopes often operate the drive and sense signals at the same frequency to allow for simple electronic circuits. However, the use of a common frequency for both functions allows the relatively powerful drive signal to inadvertently electrically couple to the relatively weak sense signal.
Residual mechanical imbalance, either stiffness or mass imbalance, of a cloverleaf micro-gyroscope results in misalignment or coupling of drive motion into the output axis. Presently, it is known to correct any misalignment of the mechanical modal axes by electronically rotating the sense and control axes into alignment with the mechanical axes., Electronic alignment is accomplished by transform circuits in the readout electronics that transform the received electrode signal axes and drive axes to the mechanical vibration axes so that a single mode at a time can be sensed and driven. Electronic tuning is achieved by means of phase adjustments in an automatic gain control circuit of the output electronics.
However, electronic alignment, in which the sense and control axes are aligned with the mechanical modal axes results in second harmonics and does not correct electronic mistuning, or asymmetry of the micro-gyroscope. Tuning is typically accomplished by AGC phase-adjustment, for example. This method has limited tuning range for high Q resonators and the tuning will change with variations in damping or temperature. It is known in the art that electrostatic tuning and AGC tuning operate by nulling quadrature amplitude. However, the quadrature amplitude signal more properly relates to misalignment so that when there is no misalignment, there is no quadrature signal, even though there may still be residual mistuning.
Further, inherent in the manufacture of a micro-gyroscope are mechanical imperfections that cause mechanical asymmetry and imbalance in the micro-gyroscope. There is a mechanical stiffness, or inertia, imbalance inherent in a micro-gyroscope that is a result of the way in which the micro-gyroscope is fabricated. This mechanical stiffness, inertia, or asymmetrical imbalance results in dynamic mechanical misalignment torques. There is a need to counteract and correct this imbalance to avoid misalignment and mistuning of the micro-gyroscope during its operation.